Derived from whirl (swirl) + pool (a small body of standing or stagnant water). The whirlpool is created in fast flowing water, spinning and pulling, so it makes sense to me that it be written "whirlpull.
'Pool' means pond/water basin pool - Dictionary of English
A whirlpool is water with a whirl (circular movement) inside it… I think it is well spelt.
Besides, the pronunciation of 'pull' is not the same as that of 'pool' (in pool a longer vowel is audible).